This invention relates to an improved method and device for measuring the time for automatic protection switching incorporated in modern communication network elements.
Modern communication networks such as the SONET standard or the SDH standard are designed to be very robust and survive failures such as fiber optic cable cuts and nodal malfunctions. Modern network elements incorporate a mechanism known as automatic protection switching which permits the network to automatically switch from a defective communication line to a protected line.
The SONET standards (ANSI T1.105 and T1.106), for example, specify that once invoked, a protection switch is required to occur in less than 50 milliseconds (with an objective of 25 milliseconds). It is important that the SONET network elements accomplish this switch time in less than the 50 millisecond specification. The protection switching protocol is not xe2x80x9chitlessxe2x80x9d and, hence, data will be lost during the process. Although 50 milliseconds is a short period of time, data loss can be significant for higher rate line speeds. Therefore, the quicker the switch, the less data loss will occur in the network. The ability to measure a network""s actual switch time is necessary for: verifying that the equipment meets the recognized standards; insuring that the protection switch will provide the level of service expected; and assessing the performance of different networks and manufacturers from a competitive standpoint.
When a protection switch occurs in a SONET network, there is a temporary interruption in the signal transported by the network. Initially, the working circuit path is good, and the test set receiver is in pattern sync. However, when a failure such as a fiber cut occurs in the network, there is a switch initiation period (the time it takes to detect the failure condition and initiate a switch). It is during the switch initiation period that the path receiver begins to first experience errors in the path monitoring section of the element and a loss of pattern is detected in a test set receiver. Within 10 milliseconds, a switch to protect should be initiated. A SONET AIS-P (path level alarm) is generated within 125 xcexcsec of detecting the failure condition. Within 50 msec, the path receiver enters an error free path condition and the test set is in pattern sync and runs error free.
Prior art methods of measuring protection switch time, known to the inventors, are either inaccurate or require multiple pieces of test equipment. For example, one method is to measure errors with a test set and calculate the error burst duration. This technique makes certain assumptions regarding the distribution of errors. The user records the bit error count recorded during a protection switch event. It is assumed that the test pattern is pseudo random and that a 50% error density is recorded due to the autocorrelation properties of the bit pattern sequence when not pattern locked. The protection switch time is calculated by multiplying the number of errors by the bit period and by doubling this amount due to the error density assumption. This method assumes that all errors are side by side; durations of no errors are not counted. In real applications, however, errors do not occur in an even distribution but rather occur in bursts separated by periods of no errors. Therefore, this method can lead to widely varied and inaccurate results.
A more accurate result under the prior art requires the manual use of multiple pieces of test equipment such as a test set with error output and an oscilloscope. The test set is used to detect the pattern bit error. The test set outputs pulses for each bit error during the protection switch event. Each pulse is recorded by the oscilloscope as a voltage transition. The user then manually measures the duration of errors from the beginning of the first error to the last error using the oscilloscope time-base. The protection switch time is the time between the first and last error recorded by the user.
Accordingly, there is a need for a method of measuring protection switch time that is accurate, automatic, and does not require multiple pieces of equipment.
An automated precision measurement device is provided for determining the protection switch time of SONET/SDH or asynchronous networks. Using digital error detection and gated counter techniques to precisely and automatically measure the error burst duration, the device provides an accurate measurement of the time required for a network element to switch from a defective line to a protected line for the purpose of delivering error free service to customer premises. The invention utilizes a timer which is started upon the detection of the first error. Data from the timer is stored upon the detection of subsequent errors to permit a precise calculation of the time between the first error present to the last error present, inclusive. If the measurement was made during a protection switch event in the network element, then the time of the error burst duration represents an accurate measurement of the protection switch time that affected the service.